1. Field of the Invention
The present invention relates to a lightweight, high-stiffness honeycomb composite panel used in aircraft, automobiles, ships, buildings and general industrial equipment and, more particularly, to a method for forming the honeycomb sandwich composite panel using an RTM (resin transfer molding) process.
2. Description of the Related Art
In a previously known method for forming a honeycomb sandwich panel, outer panels formed as skins are placed on the top and bottom surfaces of a honeycomb core respectively and are bonded to the honeycomb core with an adhesive applied therebetween under heat and pressure. In another previously known method, prepreg sheets are stacked by hand lay-up operation on the top and bottom surfaces of a honeycomb core respectively, and are bonded to the honeycomb core with an adhesive applied therebetween in a vacuum space established in a vacuum bag enclosing the prepreg sheets and the honeycomb core using a hot press or an autoclave.
In addition, Japanese Patent Laid-Open No. 295362/1997 describes a method for forming a honeycomb sandwich panel using the RTM process, in which an impregnating resin is prevented from flowing into the cells of a honeycomb core with resin/moisture barrier films formed of a particular macromolecular film.
A method for forming a honeycomb sandwich composite panel disclosed in JP-A No. 5502968/2000 uses a polyimide film, such as Kapton® commercially available from E.I. du Pont de Nemours and Company, as a bonding barrier layer to prevent the resin impregnated into sheets (prepreg sheets) from flowing into the cells of a honeycomb core when a honeycomb sandwich composite panel is formed.
A method for forming a honeycomb sandwich composite panel disclosed in JP-A No. 167950/2000 comprises the steps of heating sealing materials and dry fabrics to be bonded to the surfaces of a honeycomb core at the thermosetting temperature of the sealing materials, impregnating the dry fabric with a thermosetting resin, and hardening the thermosetting resin impregnated into the dry fabrics by pressing and heating those under predetermined conditions.
The known method for forming a honeycomb sandwich composite panel employing the RTM process needs to prevent the resin from flowing into the cells of the honeycomb core. Therefore, the cells of the honeycomb core need to be sealed before thermo-setting the honeycomb core in a mold. Thus, two steps and complicated work are necessary before starting an impregnating process, which is an impediment of cost reduction.
The method for forming a honeycomb sandwich composite panel disclosed in JP-A Nos. 2995362/1997 and 502968/2000 bond the skins and the honeycomb core together with a resin/moisture-resistant film of a specific polymer and a bonding barrier layer, respectively. Therefore, both the surfaces of the resin/moisture-resistant film and the bonding barrier layer must be coated with an adhesive, which increases the weight of the honeycomb sandwich composite panel. Generally, the specific polymer film is unsatisfactory in adhesive property and incapable of exerting high adhesive strength. Therefore, the film needs to be treated beforehand by a special pretreatment process, which increases processes and provides an impediment to cost reduction.
Although the method for forming the honeycomb sandwich composite panel disclosed in JP-A No. 167950/2000 is capable of solving the aforementioned problems, this method needs at least two thermo-setting adhesive films and epoxy resin films on the opposite sides of a carrier member, that is this method needs many sealing films.